The present invention relates to an antenna diversity system in particular to an antenna diversity system of a wireless device.
In known wireless systems, different mechanisms contribute to the propagation of a radio frequency signal. As the radiated electromagnetic waves travel from the emitter to the receiver, they encounter obstacles (like for example walls and furniture in indoor environments, or buildings, trees and vehicles in outdoor environments) and as a result some of the energy carried by the waves is absorbed, reflected, scattered and/or diffracted. Thus, not only the signal component that comes from the emitter following a direct path arrives at the receiver, but also other components of the same signal that follow either reflected, diffracted or scattered paths. However, since these other components follow longer paths, they arrive at a later time (i.e., with different phase) than the direct path. The propagation can be furthermore complicated by the fact that in some cases no direct path (or line-of-sight, LOS) will be possible between emitter and receiver.
In typical wireless systems the transmitted signal will encounter several obstacles, giving rise to a multiplicity of propagation paths, and signal components arriving at the receiver with different delays. Furthermore, since the transmitter, the receiver and the obstacles can change their position over time, the characteristics of the multipath propagation channel will be time-variant.
The multipath propagation results in the combination of several signal components with different phases at the receiving antenna. This out-of-phase addition can result in a temporary cancellation of the received signal (phenomenon known as fading), with the subsequent loss of information. This problem becomes more critical for wireless systems involving data transmission, because fading is responsible for the interruption of the communication, the loss of data (and subsequent increase in bit error rate, BER), and the decrease of the data bit rate. All these aspects degrade the quality of service (QoS) of the system.
An important technique used to overcome these impairments of the quality of communication available in the wireless channel is antenna diversity. The basic concept of diversity is to provide the receiver with more than one versions (also referred to as branches) of the transmitted signal, where each version is received through a different channel. If the channels are substantially independent (or uncorrelated), then the probability of having simultaneously a fading in all of them will be very small, which means that the signal formed from combining all the branches at the receiver will have many fewer deep fades than either one of the individual signals.
Antenna diversity is also useful in Multiple-input Multiple-Output (MIMO) systems. In such systems, a transmitter uses a first set of antennas to transmit different data streams over the same wireless propagation channel. At the receiver, a second set of antennas (wherein said second set does not need to comprise the same number of antennas as the ones in said first set) provides a MIMO detector with a plurality of received signals. Each one of these signals comprises multipath components of different transmitted data streams. A MIMO detector is able to extract from the received signals at least some of the data streams sent by the transmitter. Therefore, the use of antenna diversity in MIMO systems makes it possible to attain higher data bit rates and/or higher capacity.
There are several ways of implementing diversity using more than one antenna like space diversity, polarization diversity and radiation pattern diversity. Although these techniques can improve substantially the QoS of the system, it is difficult to implement an effective antenna diversity system in a wireless portable device (such as for instance a mobile phone, a smartphone, a PDA, a MP3 player, a headset, a USB dongle, a laptop, a PCMCIA or Cardbus 32 card) due to the reduced dimensions and form factors of current wireless devices, which will become even more critical in future devices as the trend is towards reducing even further their dimensions.
Space diversity is achieved by having at least two antennas separated in space as to obtain sufficiently low correlation between the signals received by any pair of antennas. It is known by a skilled-in-the-art person that low correlation will occur when the antennas are separated a distance of at least a half of the free-space operation wavelength of the antennas.
However, the typical dimensions of the printed circuit boards (PCB) of wireless devices makes space diversity difficult to implement in such devices and lead to a poor diversity gain (i.e., improvement in the QoS). Furthermore, the real estate requirements of several printed antennas or chip antennas (both in terms of antenna footprint and antenna clearance from ground plane) on the same PCB might be prohibitive for a typical wireless device. The problem will only aggravate as the trend is to put more functionality and services in smaller PCBs.
Polarization diversity takes advantage of the fact that the propagation phenomena in the wireless channel tend to be independent for orthogonal polarizations. This diversity technique can be implemented using two collocated antennas with orthogonal polarizations, or instead one cross-polarized antenna. Although this approach would ease the requirements of PCB area for the antenna, the shapes and form factors of real PCBs make it difficult to obtain nearly orthogonal polarizations.
Radiation pattern diversity uses directional antennas oriented to cover different angular regions of the space to obtain little correlation between the detected signals. However, as it happens with polarization diversity, the shapes and form factors of real PCBs lead to antennas with fairly omnidirectional pattern, hence resulting in poor diversity gain.
Further the invention relates to an antenna in a package or an antenna component.
The current trend in the market of wireless handheld devices, and more generally wireless portable devices, is the addition of more and more functionality and added-value services (such as for instance but not limited to internet and/or email browsing, personal organizers, geo-positioning and emergency location services, short-range connectivity with peripherals, television and/or radio receivers using DVB-H, DMB or DAB standards, MP3 player, digital cameras, or digital video recorders and/or players) into the devices, while at the same time reducing their overall dimensions.
Typically, a wireless handheld device contains a multilayer PCB which carries the electronic components, modules and other circuitry of said device. One or more layers of the multilayer PCB contain tracks that interconnect the different electronic components or modules mounted on the PCB. Other layers of said PCB are used to power the electronic components or modules and to ground them. These layers are commonly referred to as the power plane and the ground plane respectively.
A technique commonly used to mount electronic components on the PCB is the surface mount technology (SMT). An SMT component can be mounted (for example by means of soldering) directly onto a surface of the PCB without requiring fitting components with wire leads into holes in the PCB. Moreover, an SMT component is usually smaller than its leaded counterpart because it has either no leads, or smaller leads. An SMT component can have short pins, flat contacts, a matrix of balls (Ball Grid Array or BGA), terminations on the body of the component (passives), or short leads in a gull-wing formation (Quad Flat Package or QFP).
As the dimensions of a wireless handheld device or a wireless portable device are reduced, so does its PCB, requiring a high density of components on the PCB. Since SMT allows electronic components to be smaller in size and be mounted on both sides of the PCB of a handheld device, this technology has widely replaced through-hole technology in the electronics industry.
As far as the integration of the antenna into a wireless handheld device or a wireless portable device is concerned, small-sized antenna solutions requiring a small region of ground plane clearance are clearly preferred. Moreover, standard low-cost antenna solutions that can be used throughout a wide range of wireless devices with different shapes and form factors are highly desired.
In some cases, a wireless handheld device or a wireless portable device comprises an antenna printed on a layer of the multilayer PCB. However, printed antennas typically are not small in size, since their dimensions are approximately a quarter of an operating wavelength of the antenna. In addition to it, they have the disadvantage of not being modular, making it necessary to design the antenna to fit in a specific device. Therefore, for the sake of modularity, it is advantageous to embed an antenna into a standard SMT-type component featuring small dimensions and low profile, and that can be mounted on the PCB of a handheld device or a portable device.
Known SMT-type antenna components use monopole antennas or inverted-F antennas (IFAs), which despite achieving some degree of miniaturization (for instance by loading the antenna with a material with high dielectric constant) still require a ground plane clearance region around the extension of the SMT antenna component to enhance the radiation process of the antenna.
WO2004042868 discloses an integrated circuit (IC) package comprising an antenna. Although the antenna comprised in the IC package can take the form of a slot antenna, the document does not provide indication on how a conducting sheet internal to the IC package and containing the slot of a slot antenna should be connected to an external ground plane (such as for example that of a PCB) in order to ensure good grounding of said conducting sheet.
Moreover, in the case of an IC package comprising an antenna as described in WO2004042868, the antenna is fed with a radio-frequency (RF) feeding signal originating in a die also contained in the IC package (i.e., no coupling of the RF feeding signal from the outside of the IC package to the inside of said IC package is required).